ASSIT PROF. IN MECHANICAL DEPARTMENT METALLOGRAPHY PREPARED BY- Prof.JAY PUJARA ASSIT PROF. IN MECHANICAL DEPARTMENT GEC, Rajkot

PRESENTTION TOPIC INCLUDE What is Metallography ? What is Microstructure & Macrostructure ? Origins of Metallography Classification Difference between Macro examination & Micro Examination. Steps involved in Micro Scopy

What is Metallography ? Metallogrphy is the science of evaluating metal structure & its characteristics.

Microstructure is the characteristic appearance & physical arrangement of a metal as observed with microscope.

Macrostructure is the appearance & physical arrangement as observed with the naked eye or with low power magnification.

ORIGINS OF METALLOGRAPHY The critical factor in the light microscopy of metals is the surface preparation of the specimen. This is the basic insight discovered by the father metallography, Henry Clifton Sorby (Fig. 1), who was the first person to examine correctly polished and chemically etched metal samples under the microscope in 1863. Henry Clifton Sorby (1826–1908)

CLASSIFICATION OF METALLOGRAPHY Macroexamination or Macroscopy or Macrography Microexamination or Microscopy or Micrography

Micro-examination Macro-examination Involves the study of the metal structure & their alloys under microscope at magnification from 20X to 2000X & the observed structure is called microstructure. Involves the study of the metal structure & their alloys by naked eye or by low power magnification up to 15X. & the observed structure is called macrostructure.

Magnification on a microscope refers to the amount or degree to which the object observed is enlarged. It is measured by multiples, such as 2x, 4x and 10x, indicating that the object is enlarged to twice as big, four times as big or 10 times as big, respectively

USE Microscopes' magnification is useful in studying biological structures.

Flake graphite in as-cast gray iron

A Macrosector,of an upset forging, showing the flow lines.

Micro Examination Macro Examination Involves much smaller areas & brings out information which can never be revealed by macro examination. Gives broad picture of the interior of a metal by studying relatively large sectioned area

Micro Examination Macro Examination AIM : To determine chemical content of alloy To discover micro defects To Reveal structures characteristic To determine the size & shape of the crystallites To indicate quality of Heat treatment etc….. AIM : To reveal the size, form & arrangement of crystallites in cast metals. To reveal cracks appearing during certain fabrication processes. To reveal fibers in deformed metals. To reveal shrinkage porosity & gas cavities. To find cause of failure of a component part.

Micro Examination Macro Examination Need not be taken to such a high degree of surface finish & so the final stages of polishing can be omitted. Macro examination is also carried out on an etched surface. Requires proper surface preparation of the specimen. Micro Examination requires that the polished specimen surface should be etched with a suitable reagent.

Steps involved in microscopy (a) Sample preparation (b) Etching ( c) Observation under microscope & interpretation

(a) Sample Preparation involves Sampling Mounting Rough polishing Fine polishing

SAMPLING Adequate & true representative sampling leads to correct results.

MOUNTING The sample has to undergo many rounds of polishing before etching. This needs easy handling of these samples. Very small or oddly shaped samples which are difficult to hold & operate for series of polishing operations need to be mounted.

ROUGH POLSHING Polishing on Emery Paper according to degree of fineness. The fineness of emery paper increases from 0 to 0000. Every time when emery paper changed, the direction of polishing is also changed.

FINE POLISHING Special quality cloth (velvet) mounted on flat rotating dise. Abrasive particles such as alumina, diamond paste suspended in water are added in cloth. Produces mirror finish at the prepared surface.

(b) Etching Grain boundaries & various phases present in the steel. Etching distinguishes different phases & grain boundaries. This step is a must before seeing the sample under microscope.

Application of etchant It can be done either by swabbing with cotton saturated or by immersing the sample into the etchant. NITAL (Nitric acid + ethyl alcohol) are popular etchant for ferrous samples.

( c) Observation under microscope & interpretation Following are the some of types of metallurgical microscopes. Optical Microscope Scanning Electron Microscope (SEM) Transmission Electron Microscope(TEM) Auger Electron Microscope

Macroexamination Broadly, examination of metal or alloy with naked eyes or with low power lenses upto 20X magnification is called as Macroexamination or macrosopy.

Utility of Macroexamination Segregation of inclusions such as sulphides, phosphides, oxides Flow of metal the process (flow lines) Porosity Defects Examination of fractured surfaces Size, shape & distribution of inclusions

Steps in Macroexamination 1. Preparation of specimen 2. Macro-etching ( if required ) 3. Interpretation of observations

1.Preparation of specimen Sectioned specimen is made free from dust ,grease Moderate polishing done by emery paper Some specimen required further fine polishing before etching.

2.Macroetching Volume of etchant required is much larger than that of microexamination. Specimen withdrawn from the etchant is washed under water & then dried by means of warm air.

MACROSCOPIC TEST

Flow line detection Forging operation makes the metal to flow in a particular direction. The slag particle or inclusion (generally present in ferrous metal) also flow in the direction of the flow of overall metal. Flow lines indicate the flow of metal.

What is Flow line ? The discontinuous lines due to etching of these slag particles known as “flow line’’ as they indicate the flow of metal.

Flow line detection technique Surface for flow line detection is polished by emery paper according grade Prepared specimen is immersed in 50% aqueous solution of hydrochloric acid maintained at 70oC for the duration of ten min. The macroetched specimen is observed for flow lines.

Spark Test

Spark test can be considered as macro examination as the inspection is carried out by unaided eye. This test is useful un sorting the steels according to their chemical composition by observing the spark struck by the steel when brought in contact with rotating abrasive wheel such as grinder.

When steel is brought in contact with rotating abrasive wheel, due to friction between steel particles become loose. These hot particles move away in from the wheel in a trajectory called “carrier lines.”

Hot steel particles following carriers lines contain carbon Hot steel particles following carriers lines contain carbon. Carbon in particles reacts with oxygen in air to form CO2. This reaction develops the internal pressure within the particle & make them explode. Explosion is expressed as burst. Bursts for different steels have different in number ,shape & intensity.

Conclusion The basis of the test is that different metals give off sparks or particles, each having a different trajectory and form. For example, wrought or ingot iron will give off a little bundle of individual lines called a " spark picture." A carbon steel will give a line of brighter colour and will throw off a series of fine branches from this line known as  " forks," or " bursts." These are due to the presence of carbon. It will thus be seen that wrought iron can readily be distinguished from carbon steel by means of the spank given off.

Sulphur print

Procedure of sulphur print Sulphur printing reaction